The invention relates generally to methods for measuring the concentration of target chemical analytes present in a biological system. One important application of the invention involves a method for monitoring blood glucose concentrations.
A number of diagnostic tests are routinely performed on humans to evaluate the amount or existence of substances present in blood or other body fluids. These diagnostic tests typically rely on physiological fluid samples removed from a subject, either using a syringe or by pricking the skin. One particular diagnostic test entails self-monitoring of blood glucose levels by diabetics.
Diabetes is a major health concern, and treatment of the more severe form of the condition, Type I (insulin-dependent) diabetes, requires one or more insulin injections per day. Insulin controls utilization of glucose or sugar in the blood and prevents hyperglycemia which, if left uncorrected, can lead to ketosis. On the other hand, improper administration of insulin therapy can result in hypoglycemic episodes, which can cause coma and death. Hyperglycemia in diabetics has been correlated with several long-term effects of diabetes, such as heart disease, atherosclerosis, blindness, stroke, hypertension and kidney failure.
The value of frequent monitoring of blood glucose as a means to avoid or at least minimize the complications of Type I diabetes is well established. Patients with Type II (non-insulin-dependent) diabetes can also benefit from blood glucose monitoring in the control of their condition by way of diet and exercise.
Conventional blood glucose monitoring methods generally require the drawing of a blood sample (e.g., by fingerprick) for each test, and a determination of the glucose level using an instrument that reads glucose concentrations by electrochemical or colorimetric methods. Type I diabetics must obtain several fingerprick blood glucose measurements each day in order to maintain tight glycemic control. However, the pain and inconvenience associated with this blood sampling, along with the fear of hypoglycemia, has led to poor patient compliance, despite strong evidence that tight control dramatically reduces long-term diabetic complications. In fact, these considerations can often lead to an abatement of the monitoring process by the diabetic. See, e.g., The Diabetes Control and Complications Trial Research Group (1993) New Engl. J. Med. 329:977-1036.
Recently, various methods for determining the concentration of blood analytes without drawing blood have been developed. For example, U.S. Pat. No. 5,267,152 to Yang et al. describes a noninvasive technique of measuring blood glucose concentration using near-IR radiation diffuse-reflection laser spectroscopy. Similar near-IR spectrometric devices are also described in U.S. Pat. No. 5,086,229 to Rosenthal et al. and U.S. Pat. No. 4,975,581 to Robinson et al.
U.S. Pat. No. 5,139,023 to Stanley describes a transdermal blood glucose monitoring apparatus that relies on a permeability enhancer (e.g., a bile salt) to facilitate transdermal movement of glucose along a concentration gradient established between interstitial fluid and a receiving medium. U.S. Pat. No. 5,036,861 to Sembrowich describes a passive glucose monitor that collects perspiration through a skin patch, where a cholinergic agent is used to stimulate perspiration secretion from the eccrine sweat gland. Similar perspiration collection devices are described in U.S. Pat. No. 5,076,273 to Schoendorfer and U.S. Pat. No. 5,140,985 to Schroeder.
In addition, U.S. Pat. No. 5,279,543 to Glikfeld describes the use of iontophoresis to noninvasively sample a substance through skin into a receptacle on the skin surface. Glikfeld teaches that this sampling procedure can be coupled with a glucose-specific biosensor or glucose-specific electrodes in order to monitor blood glucose. Finally, International Publication No. WO 96/00110 to Tamada describes an iontophoretic apparatus for transdermal monitoring of a target substance, wherein an iontophoretic electrode is used to move an analyte into a collection reservoir and a biosensor is used to detect the target analyte present in the reservoir.
The present invention provides methods and sampling systems for measuring the concentration of an analyte present in a biological system. The methods of the invention generally entail sampling and detecting an analyte from the biological system and deriving a detectable signal therefrom, wherein the signal is specifically related to the analyte. The signal can be correlated with a measurement value indicative of the concentration of analyte present in the biological system. Sampling system configurations and/or measurement techniques are used to minimize the effect of interfering species on a particular sensing means.
Analyte sampling is carried out using a transdermal sampling system that is placed in operative contact with a skin or mucosal surface. In preferred embodiments, the sampling system transdermally extracts the analyte from the biological system using iontophoresis. The transdermal sampling system can be maintained in operative contact with the skin or mucosal surface to provide for continual or continuous analyte measurement.
The analyte can be any specific substance or component that one is desirous of detecting and/or measuring in a chemical, physical, enzymatic, or optical analysis. Such analytes include, but are not limited to, amino acids, enzyme substrates or products indicating a disease state or condition, other markers of disease states or conditions, drugs of abuse, therapeutic and/or pharmacologic agents, electrolytes, physiological analytes of interest (e.g., calcium, potassium, sodium, chloride, bicarbonate (CO2), glucose, urea (blood urea nitrogen), lactate, hematocrit, and hemoglobin), lipids, and the like. In preferred embodiments, the analyte is a physiological analyte of interest, for example glucose, or a chemical that has a physiological action, for example a drug or pharmacological agent.
Accordingly, it is an object of the invention to provide a method for measuring an analyte present in a biological system. The method entails a step for transdermally extracting the analyte from the biological system in an extraction step using a sampling system that is in operative contact with a skin or mucosal surface of the biological system; and a step for contacting the extracted analyte with a sensor means in a sensing step to obtain a detectable signal which is specifically related to the analyte. The extraction and sensing steps are conducted in a measurement cycle which selectively favors analyte-specific signal components over signal components due to interfering species.
In one aspect of the invention, a method is provided for measuring the concentration of an analyte present in a biological system. The method includes a measurement cycle which comprises an extraction step in which a sample containing the analyte is transdermally extracted from the biological system using a sampling system that is in operative contact with a skin or mucosal surface of the biological system. The method also entails a sensing step in which the extracted sample is contacted with sensor means to obtain a measurement signal that is related to analyte concentration. The measurement cycle further comprises a process for selectively favoring analyte-specific signal components over signal components due to interfering species. Such processes can include (a) a differential signal process which subtracts non-analyte signal components from the measurement signal, (b) a delay step which is performed between the extraction and sensing steps, (c) a selective electrochemical detection process which is performed during the sensing step, (d) a purge step which is performed after the sensing step, (e) a charge segregation step (as in Example 1), or any combination of the processes of (a)-(e).
In another aspect of the invention, a method is provided for measuring the concentration of an analyte present in a biological system. The method includes a measurement cycle which comprises transdermally extracting the analyte from the biological system in an extraction step using an iontophoretic sampling system that is in operative contact with a skin or mucosal surface of the biological system, and contacting the extracted analyte with a sensor means in a sensing step to obtain a detectable signal which is specifically related to the analyte. In particular, the sampling system comprises (a) a first collection reservoir containing an ionically conductive medium, a first iontophoretic sampling means for extracting substances including the analyte from the biological system into the first collection reservoir to obtain a concentration of the analyte, and a first sensor element, wherein the first sampling means and the first sensor element are in operative contact with the first collection reservoir; and (b) a second collection reservoir containing an ionically conductive medium, a second iontophoretic sampling means for extracting substances including the analyte from the biological system into the second collection reservoir, and a second sensor element, wherein the second sampling means and the second sensor element are in operative contact with the second collection reservoir. The measurement cycle comprises (a) operating the first iontophoretic sampling means as an iontophoretic cathode during a first phase of the extraction step, (b) detecting substances extracted into the first reservoir with the first sensor element during a first phase of the sensing step to obtain a first signal, (c) purging residual signal from the sampling system in a purging step, (d) operating the second iontophoretic sampling means as an iontophoretic cathode during a second phase of the extraction step, and (e) detecting substances extracted into the second reservoir with the second sensor element during a second phase of the sensing step to obtain a second signal. At least one of the first and second signals comprises an analyte-specific signal component.
In one particular embodiment, one of the collection reservoirs includes an enzyme that reacts specifically with the extracted analyte, and the second reservoir does not contain the enzyme.
In yet another aspect of the invention, a method is provided for measuring the concentration of an analyte present in a biological system. The method includes transdermally extracting the analyte from the biological system in an extraction step using an iontophoretic sampling system that comprises first and second collection reservoirs which are respectively in operative contact with first and second iontophoretic sampling means and a skin or mucosal surface of the biological system. The first and second iontophoretic sampling means extract substances including the analyte from the biological system into the first and second collection reservoirs, and the first iontophoretic sampling means is operated as a cathode during the extraction step. The method further entails passively collecting substances which diffuse from, or are secreted by the biological system into a third collection reservoir using a passive transdermal sampling system that is in operative contact with a skin or mucosal surface of the biological system. The analyte that is extracted into the first collection reservoir is then contacted with a sensing means in a sensing step to obtain an active signal, and the substances collected into the third collection reservoir are contacted with the sensing means to obtain a blank signal. The blank signal is then subtracted from the active signal to provide an analyte-specific signal.
In one aspect of the methods and devices of the present invention, a charge segregation step can be used to reduce the presence of interfering species at the sensing means. For example, the measurement cycle can include a charge segregation step wherein sensing takes place at the cathode and certain interfering species preferentially collect at the anode, or visa versa.
In a related aspect of the invention, a method is provided for measuring the concentration of an analyte present in a biological system. The method includes transdermally extracting the analyte from the biological system in an extraction step using an iontophoretic sampling system that comprises first, second, and third collection reservoirs which are respectively in operative contact with first, second, and third iontophoretic sampling means and a skin or mucosal surface of the biological system. The first, second, and third iontophoretic sampling means respectively extract substances including the analyte from the biological system into the first, second, and third collection reservoirs, and the first and second iontophoretic sampling means are operated as cathodes during the extraction step. The method further entails contacting the analyte extracted into the first collection reservoir with a sensing means in a sensing step to obtain an active signal, and contacting the substances extracted into the second collection reservoir with the sensing means to obtain a blank signal. An analyte-specific signal is then obtained by subtracting the blank signal from the active signal.
In a still further related aspect of the invention, a method is provided for measuring the concentration of an analyte present in a biological system. The method includes transdermally extracting the analyte from the biological system in an extraction step using an iontophoretic sampling system that comprises first, second, and third collection reservoirs which are respectively in operative contact with first, second, and third iontophoretic sampling means and a skin or mucosal surface of the biological system. The first, second, and third iontophoretic sampling means respectively extract substances including the analyte from the biological system into the first, second, and third collection reservoirs, and the first and second iontophoretic sampling means are operated as cathodes during the extraction step. The method further entails contacting the analyte extracted into the first and second collection reservoirs with a sensing means in a sensing step to obtain multiple analyte-specific signals.
It is another object of the invention to provide a sampling system for measuring an analyte present in a biological system. The sampling system comprises, in operative combination (a) a sampling means for extracting the analyte from the biological system, (b) a sensing means in operative contact with the analyte extracted by the sampling means, and (c) a microprocessor means in operative communication with the sampling and sensing means. The sampling means is adapted for extracting the analyte across a skin or mucosal surface of a biological system. In preferred embodiments, the sampling means is used to continually or continuously extract the analyte. The sensing means is used to obtain a detectable signal from the extracted analyte, wherein the signal is specifically related to the analyte. The microprocessor means is used to control the sampling means and the sensing means to provide for one or more measurement cycles.
In one aspect of the invention, the sampling system comprises (a) a first collection reservoir containing an ionically conductive medium, a first iontophoretic sampling means for extracting substances including the analyte from the biological system into the first collection reservoir, and a first sensor element, wherein the first sampling means and the first sensor element are in operative contact with the first collection reservoir; and (b) a second collection reservoir containing an ionically conductive medium, a second iontophoretic sampling means for extracting substances including the analyte from the biological system into the second collection reservoir, and a second sensor element, wherein the second sampling means and the second sensor element are in operative contact with the second collection reservoir. In the subject sampling system, the microprocessor controls a measurement cycle which entails (a) operating the first iontophoretic sampling means as an iontophoretic cathode during a first phase of the extraction step, (b) detecting substances extracted into the first reservoir with the first sensor element during a first phase of the sensing step to obtain a first signal, (c) purging residual signal from the sampling system in a purging step, (d) operating the second iontophoretic sampling means as an iontophoretic cathode during a second phase of the extraction step, and (e) detecting substances extracted into the second reservoir with the second sensor element during a second phase of the sensing step to obtain a second signal. At least one of the first and second signals comprises an analyte-specific signal component.
In a further aspect, the methods and devices of the present invention can include enhancement of skin permeability by pricking the skin with micro-needles when the biological system includes skin, or, for example, a mucosal surface. Such pricking with micro-needles can facilitate extraction an analyte of interest from the biological system.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following disclosure, or may be learned by practice of the invention.